1. Field of the Invention
The invention relates to an apparatus and a method for testing semiconductor memories. It relates in particular to an apparatus for testing semiconductor memories having a main cell array and having a redundant memory cell area, using a conventional test apparatus.
Test apparatuses often have their own fault memory for storing fault maps. Such a fault memory is expensive and can only ever be enlarged up to a maximum size. In turn, this limits the size and number of semiconductor memories that can be tested in parallel. The fault memory is permanently connected to specific tester inputs, in many commercial test systems.
At the same time, it is normal for the tester to be equipped with a number of inputs per chip. This number is a multiple of nine in order to allow testing of chips with a parity bit for each byte. These inputs are generally unused for the many chip types without such a parity bit.
If, as stated above, fault memory capacity is permanently connected to these inputs, it is desirable to make this fault memory usable despite this, in order to maximize the number of chips to be tested at the same time, or in order to save the procurement costs of the fault memory, which is generally in the form of high-speed SRAMs.
In addition to the main cell array, semiconductor memories normally also have a redundant memory cell area. This redundant memory cell area allows replacement of faulty memory cell areas. The replacement process is accomplished by using a laser to cut through specific fusible wires, called fuses, on the chip after testing. In consequence, a segment in which a faulty cell is located is disconnected and is replaced by a serviceable segment in the redundant memory cell area. When testing semiconductor memories, ensuring that a faulty segment in the main cell array is not replaced by a faulty segment in the redundant memory cell area is necessary. This necessitates the testing of the cells in the redundant memory cell area as well.
The size of the redundant memory cell area is generally a few percent of the main cell array. The fault memory in modern conventional test systems is, however, often configurable only in powers of two (2) per input. So, whenever the main cell array itself already fills the entire fault memory available, checking the relatively small number of redundant cells becomes necessary. This means, when using conventional methods, the test parallelity must be halved or, provided this is still possible, the tester needs to be upgraded with twice the fault memory, which is expensive.
It is accordingly an object of the invention to provide an electronic circuit, test-apparatus assembly, and method for outputting a data item that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that tests the redundant memory cell area in semiconductor memories with a minimal tester memory.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an electronic circuit for outputting contents of a memory matrix to a test apparatus. The electronic circuit includes a memory matrix having memory spaces each with an address storing an addressed data item. The addresses when stated allows the respective addressed data item to be read. Data outputs can connect to a test apparatus and output addressed data items. The electronic circuit also includes a plurality of address inputs. At least one of the address inputs specifies data outputs depending on the address of the addressed data item. An output circuit connects to at least one of the address inputs and to the data outputs, and to the memory matrix. The output circuit reads addressed data items stored in the memory matrix and outputs the read addressed data items to the specified data outputs.
With the objects of the invention in view, there is also provided an assembly for testing a memory matrix having a circuit including a memory matrix having memory spaces, each with an address storing an addressed data item. The addresses when stated allow the respective addressed data item to be read. Data outputs connect to a test apparatus and output addressed data items. The assembly also includes a plurality of address inputs. At least one of the address inputs specifies data outputs depending on the address of the addressed data item. An output circuit connects to at least one of the address inputs and to the data outputs, and to the memory matrix, reads addressed data items stored in the memory matrix, and outputs the read addressed data items to the specified data outputs. The apparatus includes a test apparatus having inputs for connecting to the data outputs of the circuit while the memory matrix outputs of the memory matrix are applied to data inputs of the circuit.
With the objects of the invention in view, there is also provided a method for outputting a data item. The first step of the method is providing a data item stored in a memory space having a memory address in the memory matrix connected to the data outputs. The data outputs connect to a test apparatus having a parity bit input. The data item in the addressed memory space is output by stating the memory address of that memory space to connected data outputs. The next step is outputting the data item the data outputs or the parity bit input of the test apparatus, depending on the memory address.
The electronic circuit according to the invention is suitable for outputting contents of a memory matrix. The electronic circuit includes address inputs, data outputs, and a memory matrix. The memory matrix is a conventional semiconductor memory, and is configured such that the memory spaces in the memory matrix can be addressed via the address inputs of the memory matrix. The electronic circuit furthermore has an output circuit, which is connected to the memory matrix, in order to transfer data stored in the memory matrix, and via which a data item can be output at the data outputs of the electronic circuit. The output circuit is connected to at least one of the address inputs and to the data outputs, in which case the data outputs on the output circuit can be connected to a test apparatus, being order to test the memory matrix. The output circuit outputs a data item stored in the memory matrix to different data outputs, depending on its address, with the process of outputting the data item via the specific data outputs or outputs (DQ0-DQ7, RDQ) being controlled via one or more of the address inputs (A0-A9, Ax). When the main cell array is being tested, the output circuit then connects the outputs of the memory matrix to the test apparatus. The redundant memory cell area is tested via one or more specific, dedicated data outputs of the circuit, which are normally applied to parity bit inputs of the test apparatus. It is thus possible to save a considerable amount of the tester""s own fault memory, or to increase the test parallelity for a given maximum fault memory size.
The electronic circuit can be provided such that the data inputs of the circuit can be produced bit-by-bit at the dedicated data output during testing of the redundant memory cell area. This production of the data inputs bit-by-bit can be controlled by a number of the address inputs. In consequence, the required size of the fault memory that is connected to the regular data inputs of the tester can be reduced. Instead, this can now be used for testing parity outputs of available memory space.
Furthermore, the invention can provide for the address inputs to have a specific address input that is used for addressing the redundant memory cell area and that signals to the circuit that it is intended to output the data to other data outputs. This minimizes the required number of address lines; as such, a specific address input is required in any case for addressing the respective memory cell area.
In one preferred embodiment, the memory matrix has no output for a parity bit. In consequence, no such parity bit is tested. As a result of which, memory space remaining in the test apparatus can be used by the apparatus of the invention of the instant application.
The dedicated data output of the circuit is preferably connected to a parity bit input of the test apparatus. The memory that is provided in the test apparatus for testing the parity bit can thus be used for testing the redundant memory cell area.
Providing for different address inputs than those used for addressing the redundant memory cell area to be used for the output at the dedicated data output is possible. This measure avoids address conflicts that would arise if both the redundant memory cell areas and the individual bits stored in them used the same addresses, or partially the same addresses.
The invention also provides for the abovementioned method for testing a memory matrix such that all the bits in the address contents of the redundant memory cell areas are output successively to the dedicated data output of the test apparatus.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an electronic circuit, test-apparatus assembly, and method for outputting a data item, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.